<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        @media screen and (max-width:639px) {
        html {
            font-size: 20px;
        }
    }
        @media screen and (min-width:640px) {
        html {
            font-size: 40px;
        }
    }
    .box {
        width: 100%;
        height: 3rem;
        background-color: green;
        font-size: 1.5rem;
        line-height: 3rem;
        color: #fff;
        text-align: center;
        font-weight: bold;
    }
    </style>
    
</head>
<body>
    <div class="box">购物车</div>
</body>
</html>